The test is of the water jacket, not the combustion chamber so the spark plug hole is not involved. Using the example gauge/valve assembly on the manifold, adapt the threads to fit a water inlet on either end on the top, plug the other one and pump up to [insert preferred pressure here] psi. 20 psi is frequently mentioned on this forum but I think that's excessive considering our cooling systems operate at much less (4 psi or so for FWC, 0 psi for RWC). I'd suggest 5~10 psi for a reasonable test.

A manual air pump is all that's needed. If you're using compressed air, be careful. This is intended as a diagnostic test, not a destructive one.

Caleb,, the pressure test is for testing the integrity of a sealed system like the water jackets at LOW PRESSURE.

Testing the cylinders is a different story as the rings are designed with a gap so they won't seal. If they do seal the top of the piston will be compromised or pulled in half. there is a test commonly called a "leak-down" test of the cylinders. This is done by injecting air into the cylinder and "listening" to the crankcase for air leakage and a guess as to the severity of the leak.

Many years ago I have my car at the shop to have some work done on it.
It was a one person shop. A couple of gents showed up with a VW engine wanting to know if they needed to tear the engine down further and replace the rings. The mechanic took the air hose to each cylinder and pressed the trigger. The verdict: The compression was fine. I assumed he could feel the resistance on his arm.
Dave N. or anybody; ever hear of anything like this?

Make sure the actual pump doesnt leak.. I made on similar to this for outdrive tests - its pointless if the pump leaks anywhere since a pump keeps the valve open when connected (of course you could pull it out quickly but that usually costs u a few psi - not good on lo pressure tests). I put my pump in a bucket of water pressurized and lo and behold - bubbles.

I bought a one way pressure activated valve on amazon for 5 bucks that I plumbed in after the schraeder valve - it makes sure air can only go one way - works like a charm!